A Bragg grating is a periodic variation in the refractive index in a small section of optical fiber cable. As light propagates along the fiber, a very narrow range of wavelengths is reflected by the Bragg grating while all other wavelengths are transmitted through the grating. The center of this reflected band is called the Bragg wavelength. In-fiber Bragg gratings sensors have been utilized as measuring devices for strain, temperature, pressure, and chemical presence, as examples. Typically, the measurements are based on identifying the peak reflection wavelength of a grating or interrogating the reflection or transmission wavelength spectrum of a grating, and then inferring strain effects based on the shifting of each grating's peak reflection wavelengths or spectra. As strain sensors, the fiber containing Bragg gratings sensors is typically bonded to the surface of a test article or is embedded within the material under test. As the test article is subjected to loading or other strain-inducing phenomena, the fiber experiences an induced strain. The induced strain at a Bragg grating sensor area causes the peak reflection wavelength of that sensor to shift, generally in a linear fashion, in relation to strain. Bragg grating sensor systems typically perform temperature measurements by inferring temperature changes after measuring fiber strain that occurs due to the fiber's coefficient of thermal expansion. Pressure and chemical sensor systems perform similarly, as they typically measure in-fiber Bragg grating sensor strain which is induced through the physical or chemical changes of materials around the sensing fiber. It is to this end that systems have been developed for the purpose of measuring or calculating the peak reflection wavelengths of in-fiber Bragg grating sensors. Because the information from a Bragg grating sensor can directly relate hazardous structural or environmental conditions to the appropriate safety interests and because properties under test can exhibit rapid changes, it is desirable to perform these wavelength measurements as quickly and as numerously as possible while maintaining acceptable accuracy.
Known methods and systems for determination of the spectra and/or the peak reflection wavelengths of multiple Bragg grating sensors in a single fiber use Optical Frequency Domain Reflectometer (OFDR) technology. Current OFDR systems derive wavelength information of Bragg gratings in a single fiber by sampling and storing the reflections from a sensing fiber while sweeping the wavelength of a system source laser. The recorded data is then manipulated through processor-intensive algorithms, such as Discrete Fourier Transforms, data filtering/smoothing, and threshold/peak detection. Such processing adds significant delay in the determination of Bragg grating reflection wavelengths, which in turn delays the measurement data repetition rates.
One known OFDR system and algorithm is capable of interrogating multiple (hundreds) Bragg grating sensors on a single fiber with the nominal wavelengths of the sensors being equal. Prior to the use of OFDR, Bragg grating sensors were interrogated using wavelength division multiplexing (WDM) systems which required the nominal reflection wavelengths of the sensors to be non-equal. WDM systems were also limited in the number of sensors they could interrogate on a single fiber. The OFDR technology allows for the interrogation of several hundred Bragg grating sensors all located on one fiber and all nominally having equal reflection wavelengths. The extensive processing of a large data set and the required analog-to-digital (A/D) conversion rates are the main limitations in the speed of the measurement using OFDR technology
Techniques which are less processor-intensive than the current OFDR method exist; however, they are limited in the number of Bragg grating sensors they are able to interrogate from one fiber. Selecting an appropriate Bragg grating sensor system usually involves assigning priority between interrogation speed, sensor number, and measurement accuracy. A system with high data repetition rates as highest priority will usually have a lower sensor quantity capability, reduced accuracy, or both. A system such as the OFDR technology, which is capable of measuring several sensors on one fiber, has high accuracy, but low data repetition rates. The most desirable system is one that combines high speed repetition rates with multiple sensor capability and accuracy comparable with other state-of-the art systems.
One method of interrogating in-fiber Bragg grating sensors using traditional Optical Frequency Domain Reflectometer (OFDR) technology is disclosed in U.S. Pat. No. 6,566,648, issued May 20, 2003, to Froggatt, the contents of which are incorporated herein in their entirety. The method disclosed in Froggatt is capable of measuring many numbers (hundreds) of Bragg grating sensors, but has undesirable processor-intensive algorithms which limit its speed capabilities. Froggatt's OFDR method utilizes a monotonically wavelength sweeping, continuous output, high coherence laser as the source for the sensing fiber and a separate fiber network which contains the necessary fiber optic components to generate calibrated interference fringes which are used to clock the sampling of the sensing fiber reflections through an A/D.
Providing the desired capability (high speed, high sensor number, and high accuracy) has been accomplished by stacking, or paralleling, multiple systems. A WDM-based system, for example, can read several Bragg grating sensors by having multiple source lasers to provide wide bandwidths, or., as is usually instituted, multiple fibers are connected to one system. An OFDR system can provide pseudo-high-speed capability and still interrogate several hundred sensors on one fiber by storing data and deriving measurements in a post-processing fashion. The latter method still does not provide useful measurements in real-time, so the high speed capability is still not completely realized. An effort to improve the processing speed of current OFDR technology through improved processing hardware and software algorithms is underway and has resulted in increased measure speeds; however, the inherent requirement of an OFDR system to sample data and perform Discrete Fourier Transforms and other related algorithms restricts the technology to be limited by A/D conversion speeds and processor speeds.